[Boatanchors] (Slightly OT) A question concerning theory behind a certain antenna matcher (Kenwood AT120)

[Phil]

Hi All,
I'm asking the question here as I don't belong to any antenna (or 
Kenwood) mail groups.  But the question could just as easily apply to a 
similar tuner used with a BA :)

Been around a long time, used various "L", "T" and even Johnson 
Matchboxes over the years.  Right now I have a Kenwood TS130S on more or 
less permanent 'loan', along with a Kenwood AT-120 mobile tuner.  Early 
next week an MFJ949E should arrive to replace it to 'match' the 88' 
ladder-line fed dipole to the rig.

According to the schematic, this critter is quite different from most 
I've used (and doesn't have nearly as wide a tuning range).  It seems to 
be centered around a classic "Pi Network" matching section, with a 
single coil tap and fixed INPUT capacitors selected per band (75, 40, 
20, 15 & 10M).  There is an adjustable capacitor from the output side of 
the coil to chassis ground (these components comprise the Pi Network). 
In addition there is a variable SERIES capacitor from the output section 
of the matching circuit to the actual output connector. The two variable 
capacitors in the output section are labeled "R Tune" and "X Tune".

Now the question.  Why would somebody design the tuner this way?  Is it 
because it was primarily designed to make for easy tune-up while mobile, 
ASSUMING that the antenna used on the mobile would already be close to 
being resonant (in other words, not need a very wide tuning range).  
That thought would be reinforced considering that there is one 
band-switch position per band and just two capacitors to adjust (no 
multiple coil taps etc to fight).  It matches the dipole VERY EASILY on 
40, 20 & 10M, but only reaches 2.5:1 on 75 and not at all on 15M, where 
other tuners have been able to match the antenna on all bands, though 
I'm sure with some losses.

Or is there another reason to choose such a circuit (such as harmonic 
reduction)?

-- 
73 de Phil,  KO6BB
http://ko6bb1.multiply.com/ (OTR Blog)
http://www.qsl.net/ko6bb/  (Web Page)

RADIOS:
Grundigs: Satellit 750 (2011), S-350 (2006)&   G6 (2011).
Kenwood: TS130S Transceiver (circa 1980).
Radio Shack: DX-380 digital portable (circa 1990).
Yaesu: Two FT-8800 Mobiles, VX8R Quad-Band (circa 2010).
Zenith: Royal-7000 Transoceanic (circa 1969).

ACCESSORIES: MFJ 1040C Pre-Selector, W9GR Audio DSP.
ANTENNA: 88' Long Ladder-line fed dipole, Apex at 35 feet.

Central California